The proposed research seeks to map the principal myocardial structural elements, myofibers and cleavage planes, using novel,noninvasive magnetic resonance imaging (MRI) methods. Cardiac mechanics is primarily the reflection of myocardial architecture, yet this architecture has been difficult to assess in the laboratory and impossible to assess in vivo. The investigators have developed a novel MRI methodology to map the architectonics of the myocardium based upon measurement of the anisotropic diffusion of myocardial water. In preliminary studies, they have shown that the orientations of these principal material anisotropies (fibers and cleavage planes) can be visualized ex vivo and in vivo using a real time stimulated echo MR technique with excellent correlation by histology. MRI methodology possess two overwhelming advantages compared with previous methods for the analysis of myocardial architecture. First, it is potentially synoptic, revealing the architecture of myocardial fibers and cleavage planes throughout the heart at the same time. Second, it is noninvasive: fully applicable to the living human subject and causing no distortion of the architecture it seeks to observe. The proposed research will validate the MR diffusion maps of myocardial architecture, using histology to establish the local correspondence between specific eigenvectors of the diffusion tensor and specific cytoarchitectonic features. A broad study of the phenomenon of diffusion measurement in a moving system, in vitro and in an isolated canine heart preparation, will then be performed. The key technical challenge addressed by this method is the detection of the microscopic motions of water molecules against the background of the vigorous macroscopic motion of the heart. The investigators will then investigate the feasibility of obtaining such data in the human subject, first optimizing acquisition strategy, next characterizing human myocardial architecture ex vivo with MRI, and last investigating the capacity of MRI to define human myocardial architecture in vivo. These measurements will afford an opportunity to address several classic questions about myocardial structure and function, and also provide a foundation for subsequent studies of cardiac pathophysiology.